Spray valve

ABSTRACT

The invention relates to an apparatus for coating a substrate ( 60 ), in particular a printed circuit board ( 61 ), with a device ( 16 ) for applying a coating material (material applying device) and a device ( 14 ) for supplying a gaseous medium (gas supplying device), the material applying device having an inner tubular element ( 16 ). The apparatus is distinguished by the fact that the gas supplying device has an outer tubular element ( 14 ) which is arranged coaxially in relation to the inner tubular element ( 16 ) and encloses the latter, so that a gas supply channel ( 19 ) is formed between the outer tubular element ( 14 ) and the inner tubular element ( 16 ), the supply channel ( 19 ) being designed in such a way that the gaseous medium flows out parallel to the coating material, in order to displace the coating material when it impinges on the substrate ( 60 ) and in this way distribute it over the surface area. Furthermore, the invention relates to a method for coating a substrate.

CROSSREFERENCE TO RELATED APPLICATION

[0001] This application claims priority of German patent application DE102 61 576, filed on Dec. 23, 2002.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to an apparatus for coating asubstrate, in particular a printed circuit board, with a device forapplying a coating material (material applying device), in particular aprotective coating, and a device for supplying a gaseous medium (gassupplying device), the material applying device having an inner tubularelement. The invention also relates to a method for coating a substratewith a coating material, preferably a printed circuit board with aprotective coating.

[0003] Apparatuses and methods of the aforementioned type are generallyknown and are used for example for providing printed circuit boards witha protective coating. A major problem when coating printed circuitboards with a protective coating is in particular that of applying theprotective coating accurately in just the right places. Small deviationsor splashes of protective coating can very quickly lead to soiling ofcontacts, which later has the effect of malfunctions of the circuit.

[0004] Generally, so-called spraying methods are often used, operatingwith differently shaped spray nozzles to atomize a coating as it leavesa nozzle. Air-assisted atomizing of the coating produces a coating filmthat is complete in the center, but depleted toward the outside, forminga spray mist and splashes. This method of spraying is particularsuitable for manual spraying by means of paint spray guns, because ofthe seamless transitions. Broad overlapping of the spray coats is aprerequisite for a uniform spray pattern, therefore no clearlydelimiting coating edge can be formed.

[0005] A coating pattern with exact delimitation of the coating area andwithout a spray mist can be obtained with so-called airless sprayingsystems. Because these require exact, accurately repeatable spraying,the spray valves used are guided by robot systems. The overlapping ofthe spray coats should in this case be as small as possible, to avoidaccumulations of material. In the prior art, different spray valves andconstructions are known.

[0006] A known method for applying a protective coating is known by thename “Select-Coat? method” and is based on a slot nozzle which has around hole on both sides. The protective coating is forced through thisnozzle. Maintaining the coating film is made possible by the two streamsof coating from the round holes. Substrates with relatively highobstacles, as represented for example by a printed circuit board, canonly be coated unsatisfactorily, because, depending on the speed andheight of the component, spray shadows form behind each obstacle passedover. Splashes also occur under these conditions. The viscosity of thecoating must not exceed 200 mPas. Otherwise, the formation of splashesincreases.

[0007] For higher-viscosity coating materials there exists a methodwhich is known as the “Swirl-Coat method” and is based on a spray jetmoved in a circular or elliptical manner. This method is well suited inthe case of printed circuit boards for areas fitted with low components,whereas greatly varying layer thicknesses are produced on areas fittedwith higher components. The application width of the sprayed area alsochanges with the unavoidably necessary changing of the distance of thenozzle from the substrate. This method is not suitable for theapplication of low-viscosity materials and coatings to printed circuitboards, because excessive differences in the height of the coating areproduced. The spraying head cannot enter between high components becauseof its size.

[0008] DE 33 29 880 A1 discloses a spray valve which has a materialnozzle, which distributes material at the nozzle outlet, and airnozzles, which direct the material stream of finely distributed dropletsthat is produced. Round or elliptical coating areas corresponding to thedirecting of the air stream are obtained and form coating paths. Thespraying head cannot enter between high components because of its size.With the resultant increasing distance, the spray jet widens andproduces splashes.

[0009] U.S. Pat. No. 6,170,760 B1 discloses an apparatus in which theair is guided around the coating material in the material nozzle in theform of an envelope. The coating is atomized as it leaves the sprayvalve. The nozzle may be made thin but not of any desired length,because otherwise the pulsation effect affecting this methodintensifies. An excessively thin nozzle (<0.6 mm) also intensifies thepulsation and produces splashes. The pulsation occurs because thecoating material tends toward the cone shape which it adopts in the airstream according to the speed, nozzle diameter and viscosity of thecoating material. The air expanding as it leaves the nozzle producessplashes if the nozzle is too thin. For this reason, a narrow coatingarea cannot be produced, but only paths about 20 mm wide. Onlylow-viscosity coatings can be handled. With materials of higherviscosity, the pulsation increases, with the consequence that splashesare produced, because both air and material accumulations form in thespray nozzle, leading to irregular material delivery.

[0010] U.S. Pat. No. 6,132,809 A discloses a rotatable dual head, whichis equipped with a dispensing head and a spraying head.

SUMMARY OF THE INVENTION

[0011] The object of the present invention is consequently to provide anapparatus and a method which are not affected by the disadvantages ofthe prior art. In particular, the apparatus and the method are intendedto permit a coating in which no splashes are produced, a thick-layeredand a thin-layered application are possible and the handling ofhigh-viscosity and low-viscosity coatings is possible without changingthe nozzle. Moreover, distribution of the coating material, for examplebehind legs of components on a printed circuit board, is also to bepossible.

[0012] The object on which the invention is based is achieved by anapparatus of the type stated at the beginning by the gas supplyingdevice having an outer tubular element which is arranged coaxially inrelation to the inner tubular element and encloses the latter, so that agas supply channel is formed between the outer tubular element and theinner tubular element, the supply channel being designed in such a waythat the gaseous medium flows out parallel to the coating material, inorder to displace the coating material when it impinges on the substrateand in this way distribute it over the surface area.

[0013] That is to say in other words that the apparatus applies thecoating material to the substrate without air being supplied, i.e. it isnot atomized, and this application is only evenly distributed on thesubstrate once it is there, by a gaseous medium, in particularcompressed air, being correspondingly supplied. The distribution of thecoating material therefore takes place evenly, since the air flows in anannular supply channel, which surrounds the stream of coating material.

[0014] This configuration readily allows the inner tubular element to bemade as long as desired, with the result that it is also possible forcoating to be carried out between relatively high components at a verysmall distance from the substrate. Consequently, it is possible to coatvirtually without any splashing.

[0015] The layer thickness itself is governed by the amount of material,the material viscosity and the intensity of the air supplied (displacingair). When the bead of material that initially forms when the coatingmaterial is applied is displaced, a roll of material is produced,coating for example component legs on a printed circuit board in anenveloping manner and also reaching component legs and components lyingin the spray shadow. Streaks forming due to any unevenness of thematerial are evened out in the case of materials which flow well, whilethey are even desired in the case of elastic materials of higherviscosity.

[0016] As already mentioned, very small distances between the substrateand the inner tubular element can be realized. This distance ispreferably 6-10 mm. As a result, relatively high contacts, such asplugs, switches, etc., are certain to remain free from splashes. Theforming of shortened round edges at corners caused by dragging of thebead is minimized by the small distance, assisted by the displacing airjet which virtually eliminates this dragging.

[0017] If, for example, low components such as surface mounted devicesare coated, the roll produced by the displacing air advances over thesecomponents, envelops them completely and ends in the form of a cleardelimitation of the coating surface that is not straight, correspondingto the additional material requirement, but free from mist and splashes.

[0018] The apparatus is suitable for coating with any flowable coatingmaterial that can be applied to the substrate from the inner tubularelement. It is also suitable for high-viscosity, elastic substances,with which greater layer thicknesses can be produced. The pressure ofthe displacing air is increased in proportion to an increase inviscosity.

[0019] In a preferred development, the inner tubular element has an endportion of equal diameter, facing the substrate.

[0020] That is to say in other words that the end portion does nottaper, as is provided for the forming of a nozzle in the prior art. Theend portion of the outer tubular element preferably has a diameter ofapproximately 4 mm and the end portion of the inner tubular element upto about 0.8 mm.

[0021] This measure has the advantage that the two tubular elements caneven enter very well between components that are very close together ona printed circuit board.

[0022] In a preferred development, a heating device is provided forheating the gaseous medium, the heated medium heating the coatingmaterial in the inner tubular element on its way through the outertubular element.

[0023] This measure has the advantage that the viscosity of thepreviously already mentioned coating material can also be kept constantin the inner tube, in that corresponding heat is supplied to it.Consequently, a reproducible coating pattern can be achieved.

[0024] In a preferred development, the outer tubular element is assigneda valve, in order to interrupt the stream of the gaseous medium throughthe element.

[0025] This measure has the effect that a thick coating in the form of amaterial ramp can be achieved, in that the displacing air is switchedoff, i.e. interrupted. Consequently, regions such as, for example, rowsof legs of ICs, etc. can be especially protected (with one and the sameconfiguration of the inner and outer tubes).

[0026] In a preferred development, the gas supplying device has anadmixing unit, in order to admix a powdered material with the gaseousmedium.

[0027] This has the advantage that the properties of the coatingmaterial can be changed.

[0028] In a preferred development, the material applying device has ashut-off valve, the closing operation of which begins and ends thematerial flow in a way corresponding to an asymptotic curve. Theshut-off valve is preferably a ball valve in the form of a three-wayvalve. It is particularly preferred if the opening and closing speed ofthe valve can be controlled.

[0029] These measures include the advantage that it is possible to agreat extent to compensate for any changing of the speed with which theapparatus is moved, occurring when it is started, each time before it isstopped and each time it changes direction.

[0030] The object on which the invention is based is also achieved by amethod of the type stated at the beginning which has the steps of:

[0031] applying the coating material to the substrate in such a way thata bead is formed, and

[0032] subjecting the bead to a jet of a gaseous medium, with the resultthat the bead is displaced and distributed over the surface area.

[0033] This method leads to the already mentioned advantages, so thereis no need for them to be repeated.

[0034] The gaseous medium is preferably guided coaxially in relation tothe coating material. The gaseous medium may be, for example, an inertgas, which has the advantage that undesired drying of the coatingmaterial on the inner tubular element is prevented.

[0035] Further advantages and refinements of the invention emerge fromthe description and the accompanying drawing.

[0036] It goes without saying that the features mentioned above andstill to be explained below can be used not only in the combinationrespectively specified but also in other combinations, or on their own,without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWING

[0037] The invention is now explained in more detail on the basis of anexemplary embodiment with reference to the drawing. In the singleFIGURE, a schematic representation of an apparatus for coating asubstrate is represented in a side view.

DETAILED DESCRIPTION OF AN PREFERRED EMBODIMENT

[0038] In the FIGURE, the nozzle region (coating head) of a coatinginstallation is represented and identified by the reference numeral 10.A coating installation is used, for example, for applying a protectivecoating to a printed circuit board, in order to protect the conductortracks from moisture, for example.

[0039] The coating installation comprises a nozzle device 12, which issubstantially made up of two tubes 14, 16. The tube 14 lies on theoutside (referred to hereafter as outer tube) and surrounds the innertube 16. The two tubes 14, 16 are arranged coaxially in relation to eachother, so that an annular region 18 is formed between the inner tube 16and the outer tube 14.

[0040] As the FIGURE reveals, the inner tube 16 ends with an offset inrelation to the outer tube 14, so that a protruding end portion 20 isobtained.

[0041] Fitted on the outer tube 14 is a tubular supply stub 22, whichopens out into the annular region 18. A gaseous medium can be introducedinto the annular region 18 via this supply stub 22. Arranged within thisannular region 18, which forms an annular channel 19, is a flow element24, which makes the flow of the gaseous medium laminar, i.e. free fromeddies, up to the open end 26 of the annular channel 19. The end 28 ofthe annular channel 19 opposite from the open end 26 is closed, so thatthe gaseous medium cannot escape at this point. Although in the FIGUREthe open end 26 of the outer tube 14 has the same diameter as theannular channel, it is also conceivable to reduce the diameter, in orderin this way to allow the outflowing air to flow more closely against theinner tube.

[0042] The inner tube 16 likewise forms a channel 30, which extendscoaxially in relation to the annular channel 19 and is likewise open atthe end portion 20. Provided at the opposite end of the inner tube 16 isa head 42, which is connected to a shut-off valve 44. Furthermore, thehead 42 is inserted or screwed into an end portion of the outer tube 14.The shut-off valve is provided for controlling the supply of a coatingmaterial into the channel 30. The gaseous medium supplied via the supplystub 22 is also controlled by means of a shut-off valve that is notrepresented. In the present exemplary embodiment, the shut-off valve 44is configured as a three-way valve, so that, apart from the coatingmaterial, a cleaning solution can also be introduced for cleaning thechannel 30.

[0043] The nozzle device 12 is attached to an arm 50, which is onlyschematically indicated in the FIGURE and allows the movement of thenozzle device 12 in several dimensions. The arrows shown in the FIGUREindicate the directions of movement. Apart from these translatorymovements, swiveling movements are also possible.

[0044] The application of a coating material to a substrate 60, forexample a printed circuit board 61, can consequently be carried out asfollows:

[0045] The coating material, for example a protective coating, is guidedinto the channel 30 of the inner tube 16 via the shut-off valve 44 froma storage container that is under air pressure and not represented inthe FIGURE. At the end portion 20, the coating material leaves in theform of a jet or in the form of drops and forms a bead on the printedcircuit board 61 when the nozzle device 12 moves in relation to theprinted circuit board 61. In order to distribute this bead over thesurface area on the printed circuit board 61, air or an inert gas isintroduced into the supply stub 22, flows through the annular channel 19and leaves at the open end 26 of the outer tube 14. On account of thecoaxial arrangement of the two tubes 14, 16, the air flows parallel tothe jet of coating material and displaces the coating material when itimpinges on the printed circuit board 61. By appropriate setting of thepressure and the amount of air, the kind of displacement on the printedcircuit board 61 can be set. The flow element 24 is given the task hereof making the air that is flowing into the annular channel free fromeddies and laminar, so that the air flows out in a laminar manner inrelation to the stream of material.

[0046] The supply of air can be interrupted by means of the shut-offvalve if it is necessary to apply a thicker layer of material to theprinted circuit board 61.

[0047] The interruption of the stream of material through the channel 30must be performed in such a way that no pressure is exerted any longeron the material located in the channel, in order in this way to preventdripping. This is possible for example by a closure being arranged atright angles to the direction of flow of the coating material.

[0048] Apart from the charging of the channel 30 with coating materialby means of a storage container that is under pressure, it is alsoconceivable to arrange a metering pump, which guides coating materialinto the channel 30 from a storage container.

[0049] On account of the small dimensions of the nozzle device 12, forexample the outside diameter of the outer tube is only 4 mm, the endportion 20 of the inner tube 16 can even be brought up very close tocomponents on the printed circuit board 61 that are arranged closetogether, for example to within 6 to 10 mm of them. This allows splashesto be avoided very effectively.

[0050] In addition, the tubes forming the channels are simplecomponents, with the result that it is possible for the nozzle device 12to be produced at low cost.

What is claimed is:
 1. An apparatus for coating a substrate, inparticular a printed circuit board, with a device for applying a coatingmaterial (material applying device) and a device for supplying a gaseousmedium (gas supplying device), the material applying device having aninner tubular element, wherein the gas supplying device has an outertubular element which is arranged coaxially in relation to the innertubular element and encloses the latter, so that a gas supply channel isformed between the outer tubular element and the inner tubular element,the supply channel being designed in such a way that the gaseous mediumflows out parallel to the coating material, in order to displace thecoating material when it impinges on the substrate and in this waydistribute it over the surface area.
 2. The apparatus as claimed inclaim 1, wherein the inner tubular element has an end portion of equaldiameter, facing the substrate.
 3. The apparatus as claimed in claim 1,wherein the end portion of the outer tubular element has a diameter ofapproximately 4 mm.
 4. The apparatus as claimed in claim 1, wherein aheating device is provided for heating the gaseous medium, the heatedmedium heating the coating material in the inner tubular element on itsway through the outer tubular element.
 5. The apparatus as claimed inclaim 1, wherein the outer tubular element is assigned a flow element,in order to make the gas stream through the element laminar.
 6. Theapparatus as claimed in claim 1, wherein the gas supplying device has anadmixing unit, in order to admix a powdered material with the gaseousmedium.
 7. The apparatus as claimed in claim 1, wherein the materialapplying device has a shut-off valve, the closing operation of whichbegins and ends the material flow in a way corresponding to anasymptotic curve.
 8. The apparatus as claimed in claim 7, wherein theshut-off valve is a ball valve in the form of a 3-way valve.
 9. Theapparatus as claimed in claim 7, wherein the opening and closing speedof the shut-off valve can be controlled.
 10. The apparatus as claimed inclaim 8, wherein the shut-off valve can be switched over for cleaningthe inner tubular element.
 11. The apparatus as claimed in claim 1,wherein it is formed in such a way that it can be made to move in atleast two directions (x axis, y axis).
 12. The apparatus as claimed inclaim 11, wherein it is designed in such a way that it can be swiveled.13. A method for coating a substrate with a coating material with thesteps of: applying the coating material to the substrate in such a waythat a bead is formed, and subjecting the bead to a jet of a gaseousmedium, with the result that the bead is displaced and distributed overthe surface area.
 14. The method of claim 13, wherein said substrate isa printed circuit board, and the coating material is a protectivecoating.
 15. The method as claimed in claim 13, wherein the gaseousmedium is guided coaxially in relation to the coating material.
 16. Themethod as claimed in claim 13, wherein the gaseous medium is an inertgas.
 17. The method as claimed in claim 15, wherein the gaseous mediumis heated, it likewise heating the coating material on the way to thesubstrate.
 18. The method as claimed in claim 13, wherein the coatingmaterial is applied to the substrate in the form of drops.
 19. Themethod as claimed in claim 13, wherein a powdered additive is admixedwith the gaseous medium.
 20. The method as claimed in claim 13, whereinlow-viscosity and high-viscosity coating materials are used.
 21. Themethod as claimed in claim 13, wherein a casting resin (hotmeltadhesive) is used as coating material.